Imaging optical system and imaging device

ABSTRACT

An imaging optical system comprises a first lens group composed of a negative single lens, a second lens group composed of a positive lens element and a negative lens element attached together, an aperture stop, a third lens group composed of a positive lens element and a negative lens element attached together, and a fourth lens group composed of a negative single lens, in this order from an object side. Expressions 0.37&lt;hF/IH&lt;0.5 and 0.37&lt;hR/IH&lt;0.5 are satisfied where “IH” denotes a maximum image height on an image plane, “hF” denotes an incident height of a principal ray, directed to a position of the maximum image height on the image plane, at its entrance into the first lens group, and “hR” denotes an exit height of the principal ray at its exit from the fourth lens group.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wide angle imaging optical systemused with a solid state image sensor, and more particularly to animaging optical system with a small outer diameter and a short totallength relative to a maximum image height and to an imaging device usingthe imaging optical system and an image sensor.

2. Description Related to the Prior Art

Various imaging devices comprising a CCD image sensor or a CMOS imagesensor and an imaging optical system are known. The imaging opticalsystem has lens groups and an aperture stop. Wide angle imaging opticalsystems are commonly used in the imaging devices for monitoring orobservation purposes. It is essential for the imaging deviceincorporated in a mobile terminal such as a mobile phone or a distal endportion of an endoscope to be compact. For example, a nasal endoscopehas been put into practical use, instead of a peroral endoscope, toreduce physical stress on a patient when the endoscope is inserted. Anouter diameter of the distal end portion of the peroral endoscope isreduced to in the order of 9 mm. An outer diameter of the distal endportion of the nasal endoscope is reduced to less than 6 mm.Accordingly, it is indispensable to downsize the imaging deviceincorporated in the distal end portion.

A cylindrical rigid tip section is provided in the distal end portion ofthe endoscope. The length of the rigid tip section is in the order of 20to 30 mm. A bendable flexible section is provided at the rear end of therigid tip section. A direction of the rigid tip section is changed byoperating an angle knob. The imaging device comprising the image sensorand the imaging optical system is incorporated in the rigid tip section.An image of a region of interest is captured through an objective windowin the rigid tip section. The rigid tip section has openings connectedto respective ends of a light guide fiber, an air/water tube, a forcepstube, and the like. Through the openings, the region of interest isilluminated, the objective window is washed or dried, and the region ofinterest is treated or a sample is taken using an appropriate tool.

It is important to shorten the rigid tip section in addition toreduction of its diameter because a longer rigid tip section increasesphysical stress on a patient and restricts bending of a flexible tubesection in a narrow body cavity. To shorten the rigid tip section, it isnecessary to make the imaging device, being a major component of therigid tip section, as thin as possible. The length of the imaging devicein an axial direction also needs to be shortened. For example, animaging optical system for an endoscope disclosed in U.S. Pat. No.4,674,844 (corresponding to Japanese Examined Patent ApplicationPublication No. 6-48327) is designed on the premise that an image sensoris used. The imaging optical system allows principal rays to be incidentobliquely on edges of an image plane. A field angle (2ω) is widened toapproximately 100° to 118° while the thickness of the imaging opticalsystem itself is reduced to 3.5-4.7 mm and the total length of theimaging optical system, between the front surface (first surface) of theimaging optical system and the image plane, is reduced to 4.2-5.5 mm.

In the wide angle imaging optical system, light rays before and afterpassing through the aperture stop are scattered. Hence, in the imagingoptical system, the light rays move away from the optical axis at anincident surface proximate to an object and at an exit surface (lastsurface) proximate to the image plane. Because the outer diameter of theimaging optical system is determined in accordance with an outerdiameter of the lens proximate to the object or the lens proximate tothe image plane, it is required to reduce the diameters of these lensesto reduce the diameter of the imaging optical system. It is necessary touse the largest image sensor possible for the imaging device to beincorporated in an extremely limited space such as the distal endportion of the endoscope. The image size of the imaging optical systemneeds to be substantially the same as an effective screen size of theimage sensor. To improve the image quality while the diameter of theimaging optical system is reduced, it is advantageous to enlarge theouter diameter of the imaging optical system to an extent not exceedingthe image size.

In view of the above, the imaging optical system disclosed in the U.S.Pat. No. 4,674,844 is not suitable for current endoscopes that requirereduction in diameter and in length, because the outer diameter is largerelative to the maximum image height corresponding to the image size,and the lens thickness (the length of the lenses in the optical axisdirection) of the entire optical system is long. The above-describedrequirements for size reduction also apply to the imaging devicesincorporated in thin mobile information terminals (PDA: Personal DigitalAssistants), typically, mobile phones.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an imaging opticalsystem with a short length between a front surface and an image planerelative to a required image size and with an outer diameter in a rangeappropriate for the image size, and to provide an imaging device havingthe imaging optical system and an image sensor.

An imaging optical system of the present invention comprises, in thisorder from an object side, a negative first lens group, a positivesecond lens group, a positive third lens group, and a negative fourthlens group. An aperture stop is provided between the second and thirdlens groups. The order of concave, convex, convex, and concave lensgroups is symmetrical about the aperture stop. The imaging opticalsystem satisfies expressions (1) to (5).

2.00<TL/IH<3.00  (1)

0.37<hF/IH<0.5  (2)

0.37<hR/IH<0.5  (3)

3.5<|f1/IH|<4.5  (4)

1.8<sum/IH<2.1  (5)

“IH” represents a maximum image height. “TL” represents a total lengthof “sum” and a back focal length. The “sum” represents the lensthickness of the entire imaging optical system. “hF” represents anincident height of a principal ray, corresponding to the maximum imageheight “IH”, at a surface proximate to the object side. “hR” representsan exit height of the principal ray at a surface proximate to an imageplane. “f1” represents a focal length of the first lens group.

By satisfying the expressions (1) to (5), the total length of theimaging optical system between the front surface proximate to the objectside and the image plane is shortened. The height of the principal rayincident on the first lens group proximate to the object side and theheight of the principal ray exiting from the fourth lens group proximateto the image plane fall within appropriate ranges relative to themaximum image height and are approximately equal to each other. Thereby,an outer diameter of the imaging optical system is reduced. This isadvantageous in correcting chromatic aberration. To correct variousaberrations, it is preferable that the lower limit of the expression (1)is 2.211.

As a specific embodiment, it is preferable that the first lens group isa single lens with a concave surface facing the image plane, and thefourth lens group is a single lens with a concave surface facing theobject side. It is preferable that at least one of the second lens groupand the third lens group is a doublet made up of a positive lens elementand a negative lens element attached together. Thereby, the chromaticaberration is corrected while other aberrations are suppressed. Thepresent invention is effectively applied to an imaging device comprisingthe imaging optical system and an image sensor such as aback-illumination type CMOS image sensor or an organic CMOS imagesensor. Photoelectric conversion efficiency of such image sensor doesnot decrease significantly even if an angle of incidence of theprincipal ray on an incident surface exceeds 30°. Particularly, thepresent invention is effective for the imaging device incorporated in adistal end portion of an endoscope for imaging a body cavity.

The outer diameter and the total length of the wide angle imagingoptical system of the present invention are small even if the fieldangle exceeds 100°. Also, the imaging optical system corrects variousaberrations including chromatic aberration, allowing production of sharpimages. Thus, the imaging optical system is effectively used for theimaging device incorporated in a distal end portion of an endoscope or amobile information terminal device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe more apparent from the following detailed description of thepreferred embodiments when read in connection with the accompanieddrawings, wherein like reference numerals designate like orcorresponding parts throughout the several views, and wherein:

FIG. 1 is a cross-sectional view of a distal end portion, in which animaging device of the present invention is incorporated, of anendoscope;

FIG. 2A is a schematic cross-sectional view of an organic CMOS imagesensor;

FIG. 2B is a schematic cross-sectional view of a back-illumination typeCMOS image sensor;

FIG. 2C is a schematic diagram illustrating an internal structure of afront-illumination type CMOS image sensor;

FIG. 3 is a characteristic diagram illustrating photoelectric conversionefficiency of the image sensors;

FIG. 4 is a graph representing spectral transmission characteristics ofan infrared cut filter;

FIG. 5 is a lens construction diagram of an optical system of an example1 of the present invention;

FIG. 6 is an aberration diagram of the example 1;

FIG. 7 is a lens construction diagram illustrating an optical system ofan example 2 of the present invention;

FIG. 8 is an aberration diagram of the example 2;

FIG. 9 is a lens construction diagram illustrating an optical system ofan example 3 of the present invention;

FIG. 10 is an aberration diagram of the example 3;

FIG. 11 is a lens construction diagram of an optical system of anexample 4 of the present invention;

FIG. 12 is an aberration diagram of the example 4;

FIG. 13 is a lens construction diagram of an optical system of anexample 5 of the present invention;

FIG. 14 is an aberration diagram of the example 5;

FIG. 15 is a lens construction diagram of an optical system of anexample 6 of the present invention;

FIG. 16 is an aberration diagram of the example 6;

FIG. 17 is a lens construction diagram of an optical system of anexample 7 of the present invention;

FIG. 18 is an aberration diagram of the example 7;

FIG. 19 is a lens construction diagram of an optical system of anexample 8 of the present invention;

FIG. 20 is an aberration diagram of the example 8;

FIG. 21 is a lens construction diagram of an optical system of anexample 9 of the present invention;

FIG. 22 is an aberration diagram of the example 9;

FIG. 23 is a lens construction diagram of an optical system of anexample 10 of the present invention;

FIG. 24 is an aberration diagram of the example 10;

FIG. 25 is a lens construction diagram of a comparative example; and

FIG. 26 is an aberration diagram of the comparative example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, holes or passages are formed in an axial direction through acylindrical rigid tip section 2 made from metal such as stainless steel.An end of an imaging device 3, an end of a light guide 4, and an end ofa forceps pipe 5 are fixed to the holes, respectively. A cap 6 is fixedso as to cover an end face of the rigid tip section 2. The cap 6 isformed with openings that are connected to the respective holes of therigid tip section 2. An opening 7 is a capture window that exposes afront face of the imaging device 3. An opening which exposes a frontface of the light guide 4 is a lighting window. A nozzle 8 is connectedto an opening in front of a water pipe. The nozzle 8 ejects cleaningwater to the front face of the imaging device 3 to clean it.

A nodal ring structure 10 is connected to a rear end of the rigid tipsection 2 through a coupling ring 9. Control force from a handlingsection of an endoscope is transmitted through a wire to the couplingring 9, and, as is well known, the nodal ring structure 10 bends inaccordance with the direction of the operation of the handling section(not shown). Thereby, the rigid tip section 2 is directed as desired.Outer circumferential surfaces of the rigid tip section 2, the couplingring 9, and the nodal ring structure 10 are covered with a flexiblewaterproof cover 11.

The imaging device 3 is composed of a metal barrel body 12, an imagingoptical system 15 and an image sensor 16. The imaging optical system 15and the image sensor 16 are incorporated in the barrel body 12. Thebarrel body 12 is fixed to surround the holes formed through the rigidtip section 2. Signal lines from a coaxial cable 17 are connected to aconnection terminal group exposed from the back of the barrel body 12.Each signal line is used for transmitting a drive signal for driving theimage sensor 16 and an image signal obtained from the image sensor 16.

The imaging device 3 is a direct-view type in which an incident surfaceof the image sensor 16 is vertical to an optical axis 15 a of theimaging optical system 15. Light rays from a last surface of the imagingoptical system 15 are incident on an image plane (that is, the incidentsurface of the image sensor 16) at various angles. An angle of incidenceof a principal ray on an edge of the image plane is larger than an angleof incidence of the principal ray at the center of the image plane. Thisapplies the same for a so-called side-view type imaging device in whichthe light rays from the last surface of the imaging optical system 15are refracted by a prism and then incident on the image sensor 16. Theimage sensor 16 may be a CCD type or a CMOS type. In view of powerconsumption and manufacturing cost, the CMOS image sensor 16 is widelyused.

In this embodiment, a CMOS image sensor is used as the image sensor 16.The CMOS image sensor (hereinafter referred to as the organic CMOSsensor) performs photoelectric conversion using an organic photoelectricconversion film. The organic CMOS sensor having a structureschematically shown in FIG. 2A is disclosed in, for example, “FUJIFILMRESEARCH & DEVELOPMENT” (No. 55-2010). For comparison, a structure ofaback-illumination type CMOS image sensor is schematically shown in FIG.2B. A structure of a front-illumination type CMOS image sensor isschematically shown in FIG. 2C. Like numerals refer to like parts inFIGS. 2A to 2C.

A micro color filter layer 18 is composed of blue (B light) transmissionfilters, green (G light) transmission filters, and red (R light)transmission filters arranged in a predetermined pattern such as a Bayerarrangement. “P” in the drawing corresponds to a pixel. The organic CMOSsensor comprises a semiconductor substrate 19 provided with a readcircuit (not shown), a circuit layer 20, a pixel electrode 24, anorganic photoelectric conversion film 22, and a transparent oppositeelectrode 23 (in this order from the bottom). The circuit layer includesnetworks such as a switching circuit and an amplification circuit. Theswitching circuit is used to read an image signal, obtained from eachpixel, through the pixel electrode 24. These circuits are electricallyconnected to each other through a connector 25 provided in the circuitlayer 20. A transparent protection layer 26 is formed over the oppositeelectrode 23. A micro color filter layer 18 covers the protection layer26.

In the back-illumination type CMOS image sensor shown in FIG. 2B,photoelectric conversion sections 28, corresponding to the respectivepixels, are provided in the semiconductor substrate 19. Eachphotoelectric conversion section 28 is made from a silicon photodiode.In the back-illumination type CMOS image sensor, a passivation film 29and the micro color filter layer 18 are layered over the semiconductorsubstrate 19. A microlens array 30 is placed over the micro color filterlayer 18 such that each pixel is covered with a microlens. The circuitlayer 20 including the switching circuit that reads out the image signalfrom each pixel is provided below the photoelectric conversion section28 (namely, on the opposite side of the light incident surface).

In the front-illumination type CMOS image sensor shown in FIG. 2C, thephotoelectric conversion section 28 made from the silicon photodiode isprovided in the semiconductor substrate 19. The circuit layer 20, thepassivation film 29, and the micro color filter layer 18 are providedover the photoelectric conversion section 28. The microlens array 30 isplaced over the micro color filter layer 18 such that each pixel iscovered with a microlens.

As shown in FIGS. 2A and 2B, in the organic CMOS sensor and theback-illumination type CMOS image sensor, a top surface of the organicphotoelectric conversion film 22 or the photoelectric conversion section28, being a light receiving surface for the photoelectric conversion, islocated above the circuit layer 20. Hence, the light receiving surfaceis provided close to an outermost surface, being the light incidentsurface. In the front-illumination type CMOS image sensor shown in FIG.2C, the top surface of the photoelectric conversion section 28, beingthe light receiving surface, is located below the circuit layer 20. Inthe organic CMOS sensor, the thickness of the organic photoelectricconversion film, which functions as the photoelectric conversionsection, is 0.5 μm. In the back-illumination type CMOS image sensor andthe front-illumination type CMOS image sensor, the thickness of thephotoelectric conversion section made from the silicon photodiode is inthe order of 5 μm in the depth direction.

In the organic CMOS sensor and the back-illumination type CMOS imagesensor, the loss of the incident light flux is reduced by placing thelight receiving surface of the photoelectric conversion section abovethe circuit layer 20. Thereby, the sensitivity of each of the organicCMOS sensor and the back-illumination type CMOS image sensor increasesas compared with that of the front-illumination type CMOS image sensor.In the organic CMOS sensor and the back-illumination type CMOS imagesensor, vignetting of the incident light by the circuit layer 20 beforethe incident light reaches the light receiving surface is prevented.Hence, sensitivity degradation caused by the light rays incident at acertain angle is prevented. When a thickness T in the depth direction ofthe organic photoelectric conversion film 22 or the photoelectricconversion section 28 is reduced, leakage of the incident light,transmitted vertically and obliquely through the micro color filterlayer 18, to the photoelectric conversion section 28 of an adjacentpixel is prevented. Thus, the occurrence of color mixing is reduced.

To prevent the color mixing, the back-illumination type CMOS sensorshown in FIG. 2B requires the microlens array 30 that allows obliquelight to be incident as vertically as possible on the micro color filterlayer 18. On the other hand, the color mixing is not likely to occur inthe organic CMOS sensor with no microlens array 30 shown in FIG. 2A,because the top surface of the organic photoelectric conversion film 22,being the light receiving surface for the photoelectric conversion, islocated close to the micro color filter layer 18.

In the back-illumination type and the front-illumination type CMOS imagesensors shown in FIGS. 2B and 2C, percentage of the light rays incidenton the photoelectric conversion section 28 of the pixel decreasesdrastically when the light rays are incident at an angle greater than orequal to 30° relative to a normal to the micro color filter layer 18even if the appropriate microlens array 30 is used. FIG. 3 shows thedecrease of the incident light using relative sensitivity. In FIG. 3,the angle of incidence of 0° corresponds to vertical incidence. In FIG.3, “M3” denotes sensitivity characteristics of a conventionalfront-illumination type CMOS image sensor. The sensitivity is reduced toin the order of 35% relative to that of the vertical incidence when theangle of incidence of the light rays is in the order of ±20°. The angleof incidence of ±30° is approximately the limit.

“M2” denotes sensitivity characteristics of the back-illumination typeCMOS image sensor. Although the sensitivity degrades when the angle ofincidence of the light rays is in the order of ±20°, as compared withthat of the vertical incidence, the sensitivity in the order of 50% ismaintained. Even at the angle of incidence of ±30°, the sensitivity inthe order of 25% is maintained, which is superior to that of thefront-illumination type CMOS image sensor. The organic CMOS sensor hassensitivity characteristics represented by “M1”, which is as high as acosine curve M0, being a theoretical limit. The organic CMOS sensor hassensitivity sufficient for practical use even at around ±45°. This isbecause the thin organic photoelectric conversion film is located closeto the light incident surface as described above.

As described above, the sensitivity characteristics of the organic CMOSsensor are significantly superior to those of the back-illumination typeCMOS image sensor. The sensitivity characteristics of the organic CMOSsensor are overwhelmingly superior to those of the front-illuminationtype CMOS image sensor. Nevertheless, it is sufficient for practical usewhen the sensitivity to the light rays at an angle of incidence of ±30°is greater than 20% relative to that of the vertical incidence. Hence,it is possible to use the back-illumination type CMOS image sensor asthe image sensor 16 of the imaging device 3 of the present invention. Byusing such image sensor, restrictions on the maximum angle of incidenceof the principal rays on the image plane are reduced. This facilitatesdesigning the imaging optical system 15 and is advantageous in narrowingthe optical system, reducing a total length of the optical system, andmaintaining high imaging performance with low aberrations, within rangessuitable for the image size.

Because the conventional CMOS image sensor is sensitive to the infraredregion, the conventional CMOS image sensor normally incorporates aninfrared cut filter in an optical system. The infrared cut filtercomprising a common multi-layer film has spectral transmissioncharacteristics depicted by T0 in FIG. 4, for example. The half themaximum transmittance, relative to the vertically incident light, is setto approximately 650 nm. Wavelength shift is caused by light raysobliquely incident on the infrared cut filter. The transmissioncharacteristics vary as depicted by T1 at 20° incidence, T2 at 30°incidence, and T3 at 40° incidence. This results in color shading. Thecolor shading causes a color of an image corresponding to a center of ascreen, on which the light rays are incident at small angles, differentfrom a color of the image corresponding to corners of the screen, onwhich the light rays are incident at large angles. To prevent the colorshading, it is necessary for the conventional CMOS image sensor toreduce the maximum angle of incidence of the principal rays on cornersof the image plane to 25°-30°.

The sensitivity of the organic CMOS image sensor decreases significantlyin the infrared region as depicted with a broken line in FIG. 4. Hence,the organic CMOS image sensor can be used without incorporating theinfrared cut filter in the optical system. With no infrared cut filter,the color shading, caused by a difference in angles of incidence oflight rays on the image plane, is prevented and the manufacturing costis reduced. Thus, it is optimum to use the organic CMOS sensor as theimage sensor 16 of the imaging device 3.

The imaging optical system 15 and the image sensor 16 are incorporatedin the barrel body 12, and fitted into and fixed to the hole providedthrough the rigid tip section 2. The image sensor 16 is incorporated inthe barrel body 12 such that the light incident surface of the imagesensor 16 is coincident with the image plane of the imaging opticalsystem 15. The image sensor 16 images a subject image formed by theimaging optical system 15. Note that a maximum image height (thedistance from the optical axis 15 a) of the subject image effectivelyformed by the imaging optical system 15 is 1.5 mm.

An outer diameter of the imaging optical system 15 incorporated in thebarrel body 12 is within an outline of the image sensor 16 so as to makethe rigid tip section 2 compact. The lens construction is determined tomake the total lens thickness or the distance between the front surface(first surface) of the imaging optical system 15 and the image planeshort while deterioration of the image quality is prevented. Basically,it is preferable to arrange a negative first lens group, a positivesecond lens group, a positive third lens group, and a negative fourthlens group in this order from an object side.

The entire optical system is downsized by satisfying the followingexpressions (1) to (3) where “IH” represents the maximum image height onthe image plane, “TL” represents the total length of the optical systemthat is the sum of the thicknesses of the lenses of the entire opticalsystem and a back focal length between the last surface and the imageplane, “hF” represents an incident height, of the principal raycorresponding to the maximum image height IH, from the optical axis 15 aat the surface proximate to the object side, and “hR” represents an exitheight, of the principal ray corresponding to the maximum image heightIH, from the optical axis 15 a at the surface proximate to the imageplane.

2.00<TL/IH<3.00  (1)

0.37<hF/IH<0.5  (2)

0.37<hR/IH<0.5  (3)

The expression (1) represents an appropriate range of the total lengthTL between the incident surface of the front lens (the lens proximate tothe object side) and the image plane. The expression (2) represents anappropriate range of the incident height hF, of the principal rayincident on the front lens and directed to a position of the maximumimage height IH on the image plane, from the optical axis 15 a at thefront lens. Hereinafter, the position of the maximum image height IH isreferred to as the maximum image height position. The expression (3)represents an appropriate range of the exit height hR, of the principalray exiting from the last lens and directed to the maximum image heightposition on the image plane, from the optical axis 15 a at the lastlens. Each of the ranges are standardized with the maximum image heighton the image plane. Namely, the expression (1) represents an appropriaterange of the total length TL of the imaging optical system when themaximum image height is 1 mm. The expression (2) represents anappropriate range of the incident height hF of the principal rayincident on the imaging optical system and directed to the maximum imageheight position, when the image with the maximum image height IH of 1 mmis formed. The expression (3) represents an appropriate range of theexit height hR of the principal ray exiting from the imaging opticalsystem and directed to the maximum image height position, when the imagewith the maximum image height IH of 1 mm is formed.

When the value TL/IH exceeds the upper limit of the expression (1), thetotal length of the optical system cannot be reduced sufficiently. Whenthe value TL/IH is less than the lower limit, correction of variousaberrations is difficult. The expressions (2) and (3) correspond tofactors which restrict the outer diameters of the first and fourth lensgroups. When the values (hF/IH, hR/IH) exceed the upper limits, theouter diameters become too large relative to the maximum image heightIH, which is disadvantageous for downsizing. When the values (hF/IH,hR/IH) are less than the lower limits, the outer diameters become toosmall so that the optical surfaces require high surface accuracy. As aresult, manufacturing cost of each lens increases.

It is effective to provide an aperture stop between the second lensgroup and the third lens group of the imaging optical system 3 so as toachieve symmetrical power distribution about the aperture stop andfacilitate correction of chromatic aberration. It is effective tosatisfy an expression (4) where f1 represents the focal length of thefirst lens group.

3.5<|f1/IH|<4.5  (4)

When the value (|f1/IH|) exceeds the upper limit of the expression (4),the negative power of the first lens group becomes too small, so thatdownsizing is difficult when the wide angle is achieved. When the value(|f1/IH|) is less than the lower limit of the expression (4), thenegative power becomes too large, so that the correction of aberration,particularly, correction of a tilt of the image plane is difficult.

It is effective to satisfy an expression (5) where “sum” denotes thethickness of the lenses between the surface, proximate to the objectside, of the first lens group and the surface, proximate to the imageplane, of the fourth lens group.

1.8<sum/IH<2.1  (5)

When the value “sum/IH” exceeds the upper limit of the expression (5),the lens thickness becomes too large so that the total length of theoptical system cannot be reduced sufficiently. When the value “sum/IH”is less than the lower limit, correction of astigmatism is difficult. Inconsideration of the correction of various aberrations including thechromatic aberration, it is preferable that at least one of the secondand third lens groups of positive power is composed of a doublet made upa positive lens element and a negative lens element attached together.The aperture stop is provided between the second and third lens groups.

Hereinafter, referring to data (optical system data) of the opticalsystem, lens construction diagrams, and aberration diagrams, examples 1to 10 of the imaging optical systems 15 are described. In the drawingsshowing the lens constructions, “G1” denotes the first lens group. “G2”denotes the second lens group. “G3” denotes the third lens group. “G4”denotes the fourth lens group. The first to fourth lens groups G1 to G4are arranged in this order from the object side. “S” denotes theaperture stop. “IP” denotes the image plane. In each example, theimaging optical system 15 has concave-convex-convex-concave powerdistribution, from the first lens group. The aperture stop S is providedbetween the second and third lens groups. The order of the concave andconvex lens groups is symmetrical about the aperture stop. In theexamples in which the second or third lens group is composed of thedoublet, “G2 a” and “G2 b”, or “G3 a” and “G3 b” denote the respectivesingle lens elements attached together. Note that each of the first lensgroup G1 and the fourth lens group G4 is composed of a single lens.Alternatively, at least one of the first and fourth lens groups G1 andG4 may be composed of a doublet.

For each surface number assigned from the object side, the opticalsystem data shows a curvature radius “r” and a distance “d” between thesurfaces each in the unit of mm. The values other than field angles(unit: degree) and exit angles (unit: degree) are also shown in the unitof mm. In each spherical aberration diagram, “F”, “d”, and “C” denoteaberration characteristics corresponding to wavelengths of spectrallines F (486.1 nm), d (587.6 nm), and C (656.3 nm), respectively. Ineach astigmatism diagram, “s” and “t” denote sagittal and tangentialaberration characteristics, respectively. Note that the back length BLdenotes the distance between the last surface of the lens and the imageplane when the object distance is infinite.

Generally, the object distance of the optical system for the endoscopeis short as compared with that of a common optical system. The imagesensor (the image plane) is moved toward the back such that the optimumfocal position of the optical system corresponds to the predeterminedobject distance suitable for observation using the endoscope. Namely,the optical system for the endoscope is used with the back length longerthan that for the infinite object distance. Shorter the object distance,the larger the amount of shift of the image plane toward the back. Theapproximate shift amount is obtained by dividing the square of the focallength by the object distance. The aberration diagram in each example isobtained with the objective distance of 10 mm.

For each of the total length TL of the optical system, the incidentheight hF, the exit height hR, the absolute value |f1| of the focallength f1 of the first lens group, and the lens thickness “sum” betweenthe front surface of the first lens group and the last surface of thefourth lens group, a value standardized with the maximum image height IHis also shown with an asterisk “*” prefixed, for example “*TL”. Theincident height hF indicates the height of the principal ray, to beincident on the maximum image height position on the image plane, at aposition of incidence on the incident surface of the first lens groupfrom the optical axis. The exit height hR indicates the height of theprincipal ray, to be incident on the maximum image height position onthe image plane, at an exit position on the exit surface of the fourthlens group from the optical axis.

Example 1

The imaging optical system 15 has a lens construction shown in FIG. 5.The optical system data is shown in table 1. The incident height [hF] atthe front surface, the exit height [hR] on the last surface, and themaximum image height IH shown in the table 1 indicate the respectiveheights from the optical axis 15 a as illustrated in FIG. 5 by way ofexample.

TABLE 1 Surface No. r d nd νd 1 4.000 0.300 1.88300 40.80 f1 −0.621 20.465 0.170 f2 0.641 3 0.667 0.680 1.83481 42.71 f3 3.122 4 −0.667 0.2601.80518 25.42 f4 −1.404 5 −1.000 0.000 fF 0.989 6 aperture 0.030 D11.410 stop fR −2.677 7 0.000 0.680 1.83481 42.71 D2 1.440 8 −0.667 0.2601.80518 25.42 9 −2.900 0.170 10  −0.675 0.300 1.88300 40.80 11  −1.790focal length [f] 1.257 back length [BL] 0.501 lens thickness [sum] 2.850total length [TL] 3.351 field angle [2ω](°) 108.5 exit angle [2δ](°)95.5 incident height [hF] 0.598 exit height [hR] 0.635 maximum imageheight 1.500 [IH] value standardized with image height *TL 2.234 *hF0.399 *hR 0.423 *|f1| 0.414 *sum 1.900

In tables 1 to 10, “f1” denotes the focal length of the first lensgroup, “f2” denotes the focal length of the second lens group, “f3”denotes the focal length of the third lens group, and “f4” denotes thefocal length of the fourth lens group. “fF” denotes the focal length ofa front lens group. “D1” denotes the distance between the front surfaceand the aperture stop. “fR” denotes the focal length of a rear lensgroup. “D2” denotes the distance between the aperture stop and the lastsurface.

In the optical system of the example 1, the second lens group G2 iscomposed of a doublet made up of a positive lens element G1 a and anegative lens element G2 b arranged in this order from the object side.The third lens group G3 is composed of a doublet made up of a positivelens element G3 a and a negative lens element G3 b arranged in thisorder from the object side. In total, the optical system is composed ofsix lenses in four groups. The total length TL between the front surfaceof the first lens group G1 and the image plane IP is 3.351 mm, the lensthickness “sum” is 2.850 mm, the incident height hF at the front surfaceis 0.598 mm, and the exit height hR at the last surface is 0.635 mm,when the maximum image height IH is 1.500 mm. Hence, it is sufficientthat the calculated outer diameter of each of the first and fourth lensgroups G1 and G4, which determines the outer diameter of the imagingoptical system 15, be 1.5 mm.

Thus, the outer diameter of the imaging optical system 15 issufficiently small relative to the screen size (in the order of 3×3 mm)of the image sensor 16. The imaging optical system 15 is suitably usedfor the wide-angle imaging device 3 incorporated in the rigid tipsection 2 of the endoscope even if the actual diameter of the imagingoptical system 15 becomes larger with the use of a flange extendingoutside of the effective diameter, when suitability of the imagingoptical system 15 to be incorporated into the barrel body 12 andsuitability for manufacture of each of the lens groups including thesecond and third lens groups G2 and G3 are taken into consideration.

The value “*TL” obtained by standardizing the total length TL with themaximum image height IH, the value “*hF” obtained by standardizing theincident height hF with the maximum image height IH, the value “*hR”obtained by standardizing the exit height hR with the maximum imageheight IH, and the value “sum” obtained by standardizing the lensthickness “sum” with the maximum image height IH satisfy the conditionsspecified by the expressions (1), (2), (3), and (5), respectively. Thevalue “*|f1|”, obtained by standardizing the absolute value |f1| of thefocal length f1 of the first lens group G1 with the maximum image heightIH, is within the range defined by the expression (4). Hence, theimaging optical system ensures the field angle 2ω of 137° whilecorrecting various aberrations as shown in FIG. 6. The imaging opticalsystem is widely used for various imaging devices which require widefield angles in addition to endoscopes.

Example 2

The imaging optical system 15 has a lens construction composed of fivelenses in four groups as shown in FIG. 7. The optical system data isshown in table 2. The aberration characteristics are shown in FIG. 8.

TABLE 2 Surface No. r d nd νd 1 3.850 0.300 1.88300 40.80 f1 −0.634 20.471 0.170 f2 0.645 3 0.667 0.940 1.83481 42.71 f3 3.212 4 −1.000 0.000f4 −1.805 5 aperture 0.030 fF 0.992 stop D1 1.410 6 0.000 0.680 1.8348142.71 fR −4.151 7 −0.667 0.260 1.80518 25.42 D2 1.440 8 −2.997 0.170 9−0.672 0.300 1.88300 40.80 10  −1.405 focal length [f] 1.191 back length[BL] 0.466 lens thickness [sum] 2.85 total length [TL] 3.316 field angle[2ω](°) 119.6 exit angle [2δ](°) 93.3 incident height [hF] 0.665 exitheight [hR] 0.669 maximum image height 1.500 [IH] value standardizedwith image height *TL 2.211 *hF 0.443 *hR 0.446 *|f1| 0.423 *sum 1.900

The optical system has a lens construction composed of five lenses infour groups. The third lens group G3 is composed of a doublet made up ofa positive lens element G3 a and a negative lens element G3 b. The totallength TL of the optical system is 3.316 mm, the lens thickness “sum” is2.85 mm, the incident height hF is 0.665 mm, the exit height hR is 0.669mm, when the maximum image height IH is 1.5 mm. Hence, the total lengthand the outer diameter fall within appropriate ranges in a mannersimilar to the example 1. Thus, the wide angle imaging optical system,suitable for an observation optical system of the endoscope, isobtained. The values “*TL” obtained by standardizing the total length TLwith the maximum image height IH, “*hF” obtained by standardizing theincident height hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “*sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Example 3

The imaging optical system 15 has a lens construction composed of fivelenses in four groups as shown in FIG. 9. The optical system data isshown in table 3. The aberration characteristics are shown in FIG. 10.

TABLE 3 Surface No. r d nd νd 1 0.000 0.300 1.88300 40.80 f1 −0.593 20.524 0.180 f2 0.785 3 0.893 0.950 1.83481 42.71 f3 1.117 4 −1.272 0.000f4 −1.063 5 aperture 0.030 fF 1.827 stop D1 1.430 6 4.425 0.680 1.8348142.71 fR 7.552 7 −0.375 0.260 1.80518 25.42 D2 1.430 8 −1.135 0.160 9−0.613 0.300 1.88300 40.80 10  −2.173 focal length [f] 1.209 back length[BL] 0.88 lens thickness [sum] 2.86 total length [TL] 3.74 field angle[2ω](°) 137.6 exit angle [2δ](°) 81.3 incident height [hF] 0.597 exitheight [hR] 0.553 maximum image height 1.500 [IH] value standardizedwith image height *TL 2.493 *hF 0.398 *hR 0.369 *|f1| 0.395 *sum 1.907

Similar to the example 2, the optical system has a lens constructioncomposed of five lenses in four groups. The third lens group G3 iscomposed of a doublet made up of a positive lens element G3 a and anegative lens element G3 b. The total length TL of the optical system is3.74 mm, the lens thickness “sum” is 2.86 mm, the incident height hF is0.597 mm, the exit height hR is 0.553 mm, when the maximum image heightIH is 1.5 mm. Hence, the total length and the outer diameter fall withinappropriate ranges in a manner similar to the above examples. Thus, thewide angle imaging optical system, suitable for an observation opticalsystem of the endoscope, is obtained. The values “*TL” obtained bystandardizing the total length TL with the maximum image height IH,“*hF” obtained by standardizing the incident height hF with the maximumimage height IH, “*hR” obtained by standardizing the exit height hR withthe maximum image height IH, “*|f1|” obtained by standardizing theabsolute value |f1| of the focal length f1 of the first lens group G1with the maximum image height IH, and “*sum” obtained by standardizingthe lens thickness “sum” with the maximum image height IH satisfy theconditions specified by the expressions (1) to (5), respectively. Thus,the imaging optical system is sufficiently downsized relative to theimage size and various aberrations are corrected.

Example 4

The imaging optical system 15 has a lens construction composed of sixlenses in four groups as shown in FIG. 11. The optical system data isshown in table 4. The aberration characteristics are shown in FIG. 12.

TABLE 4 Surface No. r d nd νd 1 0.000 0.280 1.88300 40.80 f1 −0.579 20.511 0.180 f2 0.751 3 0.923 0.600 1.80518 25.42 f3 1.059 4 0.480 0.3001.83481 42.71 f4 −0.975 5 −1.124 0.000 fF 1.457 6 aperture 0.030 D11.360 stop fR 20.834 7 0.000 0.700 1.83481 42.71 D2 1.510 8 −3.232 0.3001.80518 25.42 9 −0.860 0.180 10  −0.655 0.300 1.92286 18.90 11  −2.936focal length [f] 1.194 back length [BL] 0.918 lens thickness [sum] 2.87total length [TL] 3.788 field angle [2ω](°) 139.1 exit angle [2δ](°)80.1 incident height [hF] 0.569 exit height [hR] 0.561 maximum imageheight 1.500 [IH] value standardized with image height *TL 2.525 *hF0.379 *hR 0.374 *|f1| 0.386 *sum 1.913

The optical system has a lens construction composed of six lenses infour groups. The second lens group G2 is composed of a doublet made upof a negative lens element G2 a and a positive lens element G2 b. Thethird lens group G3 is composed of a doublet made up of a positive lenselement G3 a and a positive lens element G3 b. The total length TL ofthe optical system is 3.788 mm, the lens thickness “sum” is 2.87 mm, theincident height hF is 0.569 mm, the exit height hR is 0.561 mm, when themaximum image height IH is 1.5 mm. Hence, the total length and the outerdiameter fall within appropriate ranges in a manner similar to the aboveexamples. Thus, the wide angle imaging optical system, suitable for anobservation optical system of the endoscope, is obtained. The values“*TL” obtained by standardizing the total length TL with the maximumimage height IH, “*hF” obtained by standardizing the incident height hFwith the maximum image height IH, “*hR” obtained by standardizing theexit height hR with the maximum image height IH, “|f1|” obtained bystandardizing the absolute value |f1| of the focal length f1 of thefirst lens group G1 with the maximum image height IH, and “sum” obtainedby standardizing the lens thickness “sum” with the maximum image heightIH satisfy the conditions specified by the expressions (1) to (5),respectively. Thus, the imaging optical system is sufficiently downsizedrelative to the image size and various aberrations are corrected.

Example 5

The imaging optical system 15 has a lens construction composed of sixlenses in four groups as shown in FIG. 13. The optical system data isshown in table 5. The aberration characteristics are shown in FIG. 14.

TABLE 5 Surface No. r d nd νd 1 6.032 0.300 1.88300 40.80 f1 −0.602 20.477 0.195 f2 0.692 3 0.741 0.702 1.80610 40.92 f3 5.095 4 −0.597 0.2511.84666 23.78 f4 −3.237 5 −0.930 0.000 fF 1.029 6 aperture 0.030 D11.448 stop fR −8.265 7 133.500 0.800 1.75500 52.32 D2 1.530 8 −0.5530.250 1.84666 23.78 9 −2.424 0.150 10  −0.802 0.300 1.51742 52.43 11 −1.735 focal length [f] 1.19 back length [BL] 0.66 lens thickness [sum]2.978 total length [TL] 3.638 field angle [2ω](°) 136.5 exit angle[2δ](°) 81.3 incident height [hF] 0.68 exit height [hR] 0.687 maximumimage height 1.500 [IH] value standardized with image height *TL 2.425*hF 0.453 *hR 0.458 *|f1| 0.401 *sum 1.985

The optical system has a lens construction composed of six lenses infour groups. The second lens group G2 is composed of a doublet made upof a positive lens element G2 a and a negative lens element G2 b. Thethird lens group G3 is composed of a doublet made up of a positive lenselement G3 a and a negative lens element G3 b. The total length TL ofthe optical system is 3.638 mm, the lens thickness “sum” is 2.978 mm,the incident height hF is 0.680 mm, the exit height hR is 0.687 mm, whenthe maximum image height IH is 1.5 mm. Hence, the total length and theouter diameter fall within appropriate ranges in a manner similar to theabove examples. Thus, the wide angle imaging optical system, suitablefor an observation optical system of the endoscope, is obtained. Thevalues “*TL” obtained by standardizing the total length TL with themaximum image height IH, “*hF” obtained by standardizing the incidentheight hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“*|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Example 6

The imaging optical system 15 has a lens construction composed of sixlenses in four groups as shown in FIG. 15. The optical system data isshown in table 6. The aberration characteristics are shown in FIG. 16.

TABLE 6 Surface No. r d nd νd 1 12.539 0.300 1.88300 40.80 f1 −0.583 20.489 0.175 f2 0.684 3 0.859 0.386 1.73800 32.26 f3 4.578 4 0.677 0.6091.88300 40.80 f4 −2.756 5 −0.979 0.000 fF 0.935 6 aperture 0.030 D11.470 stop fR −5.547 7 −7.425 0.800 1.80100 34.97 D2 1.530 8 −0.5990.250 1.92286 18.90 9 −1.864 0.150 10  −0.807 0.300 1.64769 33.79 11 −1.688 focal length [f] 1.247 back length [BL] 0.903 lens thickness[sum] 3.00 total length [TL] 3.903 field angle [2ω](°) 136.5 exit angle[2δ](°) 73.1 incident height [hF] 0.628 exit height [hR] 0.623 maximumimage height 1.500 [IH] value standardized with image height *TL 2.602*hF 0.419 *hR 0.415 *|f1| 0.389 *sum 2.000

The optical system has a lens construction composed of six lenses infour groups. The second lens group G2 is composed of a doublet made upof a negative lens element G2 a and a positive lens element G2 b. Thethird lens group G3 is composed of a doublet made up of a positive lenselement G3 a and a negative lens element G3 b. The total length TL ofthe optical system is 3.903 mm, the lens thickness “sum” is 3.00 mm, theincident height hF is 0.628 mm, the exit height hR is 0.623 mm, when themaximum image height IH is 1.5 mm. Hence, the total length and the outerdiameter fall within appropriate ranges in a manner similar to the aboveexamples. Thus, the wide angle imaging optical system, suitable for anobservation optical system of the endoscope, is obtained. The values“*TL” obtained by standardizing the total length TL with the maximumimage height IH, “*hF” obtained by standardizing the incident height hFwith the maximum image height IH, “*hR” obtained by standardizing theexit height hR with the maximum image height IH, “*|f1|” obtained bystandardizing the absolute value |f1| of the focal length f1 of thefirst lens group G1 with the maximum image height IH, and “sum” obtainedby standardizing the lens thickness “sum” with the maximum image heightIH satisfy the conditions specified by the expressions (1) to (5),respectively. Thus, the imaging optical system is sufficiently downsizedrelative to the image size and various aberrations are corrected.

Example 7

The imaging optical system 15 has a lens construction composed of fivelenses in four groups as shown in FIG. 17. The optical system data isshown in table 7. The aberration characteristics are shown in FIG. 18.

TABLE 7 Surface No. r d nd νd 1 0.000 0.300 1.88300 40.80 f1 −0.592 20.523 0.200 f2 0.732 3 0.714 0.800 1.80610 40.92 f3 1.427 4 −1.706 0.000f4 −1.763 5 aperture 0.030 fF 2.036 stop D1 1.300 6 2.780 0.352 1.8466623.78 fR 4.378 7 0.600 0.683 1.77030 47.40 D2 1.530 8 −1.299 0.165 9−0.686 0.300 1.67300 38.15 10  −1.913 focal length [f] 1.211 back length[BL] 0.734 lens thickness [sum] 2.83 total length [TL] 3.564 field angle[2ω](°) 136.3 exit angle [2δ](°) 79.6 incident height [hF] 0.581 exitheight [hR] 0.663 maximum image height 1.500 [IH] value standardizedwith image height *TL 2.376 *hF 0.387 *hR 0.442 *|f1| 0.395 *sum 1.887

The optical system has a lens construction composed of five lenses infour groups. The third lens group G3 is composed of a doublet made up ofa negative lens element G3 a and a positive lens element G3 b. The totallength TL of the optical system is 3.564 mm, the lens thickness “sum” is2.83 mm, the incident height hF is 0.581 mm, the exit height hR is 0.663mm, when the maximum image height IH is 1.5 mm. Hence, the total lengthand the outer diameter fall within appropriate ranges in a mannersimilar to the above examples. Thus, the wide angle imaging opticalsystem, suitable for an observation optical system of the endoscope, isobtained. The values “*TL” obtained by standardizing the total length TLwith the maximum image height IH, “*hF” obtained by standardizing theincident height hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “*sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Example 8

The imaging optical system 15 has a lens construction composed of fivelenses in four groups as shown in FIG. 19. The optical system data isshown in table 8. The aberration characteristics are shown in FIG. 20.

TABLE 8 Surface No. r d nd νd 1 9.605 0.300 1.88300 40.80 f1 −0.565 20.467 0.200 f2 0.688 3 0.782 0.900 1.80400 46.57 f3 4.737 4 −0.922 0.000f4 −3.699 5 aperture 0.030 fF 1.058 stop D1 1.400 6 0.000 0.800 1.7200050.23 fR −12.895 7 −0.600 0.266 1.92286 20.88 D2 1.569 8 −1.759 0.173 9−0.818 0.300 1.58144 40.75 10  −1.498 focal length [f] 1.241 back length[BL] 0.862 lens thickness [sum] 2.969 total length [TL] 3.831 fieldangle [2ω](°) 137.9 exit angle [2δ](°) 71.3 incident height [hF] 0.624exit height [hR] 0.659 maximum image height 1.500 [IH] valuestandardized with image height *TL 2.554 *hF 0.416 *hR 0.439 *|f1| 0.376*sum 1.979

The optical system has a lens construction composed of five lenses infour groups. The third lens group G3 is composed of a doublet made up ofa positive lens element G3 a and a negative lens element G3 b. The totallength TL of the optical system is 3.831 mm, the lens thickness “sum” is2.969 mm, the incident height hF is 0.624 mm, the exit height hR is0.659 mm, when the maximum image height IH is 1.5 mm. Hence, the totallength and the outer diameter fall within appropriate ranges in a mannersimilar to the above examples. Thus, the wide angle imaging opticalsystem, suitable for an observation optical system of the endoscope, isobtained. The values “*TL” obtained by standardizing the total length TLwith the maximum image height IH, “*hF” obtained by standardizing theincident height hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “*sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Example 9

The imaging optical system 15 has a lens construction composed of sixlenses in four groups as shown in FIG. 21. The optical system data isshown in table 9. The aberration characteristics are shown in FIG. 22.

TABLE 9 Surface No. r d nd νd 1 9.814 0.300 1.88300 40.80 f1 −0.565 20.468 0.161 f2 0.702 3 1.054 0.795 1.91082 35.25 f3 3.657 4 −0.579 0.2681.92286 20.88 f4 −2.378 5 −0.841 0.000 fF 0.905 6 aperture 0.030 D11.524 stop fR −5.147 7 −4.258 0.724 1.75501 51.16 D2 1.475 8 −0.5560.250 1.92286 20.88 9 −1.356 0.171 10  −0.955 0.300 1.58144 40.75 11 −3.443 focal length [f] 1.249 back length [BL] 1.101 lens thickness[sum] 3.00 total length [TL] 4.10 field angle [2ω](°) 137.5 exit angle[2δ](°) 70.2 incident height [hF] 0.622 exit height [hR] 0.580 maximumimage height 1.500 [IH] value standardized with image height *TL 2.733*hF 0.415 *hR 0.387 *|f1| 0.377 *sum 2.000

The optical system has a lens construction composed of six lenses infour groups. The second lens group G2 is composed of a doublet made upof a positive lens element G2 a and a negative lens element G2 b. Thethird lens group G3 is composed of a doublet made up of a positive lenselement G3 a and a negative lens element G3 b. The total length TL ofthe optical system is 4.10 mm, the lens thickness “sum” is 3.00 mm, theincident height hF is 0.622 mm, the exit height hR is 0.580 mm, when themaximum image height IH is 1.50 mm. Hence, the total length and theouter diameter fall within appropriate ranges in a manner similar to theabove examples. Thus, the wide angle imaging optical system, suitablefor an observation optical system of the endoscope, is obtained. Thevalues “*TL” obtained by standardizing the total length TL with themaximum image height IH, “*hF” obtained by standardizing the incidentheight hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“*|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Example 10

The imaging optical system 15 has a lens construction composed of sixlenses in four groups as shown in FIG. 23. The optical system data isshown in table 10. The aberration characteristics are shown in FIG. 24.

TABLE 10 Surface No. r d nd νd 1 17.339 0.300 1.88300 40.80 f1 −0.596 20.507 0.168 f2 0.730 3 0.838 0.800 1.83400 37.34 f3 29.651 4 −0.6000.250 1.92286 20.88 f4 −10.519 5 −0.899 0.000 fF 1.053 6 aperture 0.030D1 1.518 stop fR −13.768 7 −6.690 0.752 1.61772 49.81 D2 1.482 8 −0.6000.250 1.92286 20.88 9 −1.580 0.150 10  −6.732 0.300 1.64000 60.08 11 0.000 focal length [f] 1.272 back length [BL] 1.045 lens thickness [sum]3.00 total length [TL] 4.045 field angle [2ω](°) 137.6 exit angle[2δ](°) 70 incident height [hF] 0.646 exit height [hR] 0.652 maximumimage height 1.500 [IH] value standardized with image height *TL 2.697*hF 0.431 *hR 0.435 *|f1| 0.398 *sum 2.000

The optical system has a lens construction composed of six lenses infour groups. The second lens group G2 is composed of a doublet made upof a positive lens element G2 a and a negative lens element G2 b. Thethird lens group G3 is composed of a doublet made up of a positive lenselement G3 a and a negative lens element G3 b. The total length TL ofthe optical system is 4.045 mm, the lens thickness “sum” is 3.00 mm, theincident height hF is 0.646 mm, the exit height hR is 0.652 mm, when themaximum image height IH is 1.50 mm. Hence, the total length and theouter diameter fall within appropriate ranges in a manner similar to theabove examples. Thus, the wide angle imaging optical system, suitablefor an observation optical system of the endoscope, is obtained. Thevalues “*TL” obtained by standardizing the total length TL with themaximum image height IH, “*hF” obtained by standardizing the incidentheight hF with the maximum image height IH, “*hR” obtained bystandardizing the exit height hR with the maximum image height IH,“*|f1|” obtained by standardizing the absolute value |f1| of the focallength f1 of the first lens group G1 with the maximum image height IH,and “*sum” obtained by standardizing the lens thickness “sum” with themaximum image height IH satisfy the conditions specified by theexpressions (1) to (5), respectively. Thus, the imaging optical systemis sufficiently downsized relative to the image size and variousaberrations are corrected.

Note that FIG. 25 shows a comparative example 1 using an optical systemdisclosed in Example 1 of U.S. Pat. No. 4,674,844 (corresponding toJapanese Examined Patent Application Publication No. 6-48327). Thecomparative example 1 is similar to the above examples 1 to 10 of thepresent invention in that the optical system is composed of four groups,first lens group G1, second lens group G2, third lens group G3, andfourth lens group G4, arranged in this order from the object side, withthe concave-convex-convex-concave power distribution and that theaperture stop S is provided between the second lens group G2 and thethird lens group G3. The aberration characteristics of the imagingoptical system of the comparative example 1 are shown in FIG. 26.

Table 11 shows optical system data of the comparative examples includingthe comparative example 1 in FIG. 25. The optical system data of thecomparative examples 2 to 7 in the table 11 corresponds to that of theimaging optical system of Examples 2 to 7 in the U.S. Pat. No.4,674,844.

TABLE 11 CE. 1 CE. 2 CE. 3 CE. 4 CE. 5 CE. 6 CE. 7 [f] 1.00 1.00 1.001.00 1.00 1.003 1.00 [BL] 0.668 0.419 0.277 0.377 0.384 0.671 0.801[sum] 3.617 3.982 3.924 3.694 4.069 4.574 4.657 [*sum] [3.6026] [3.2904][3.2936] [3.6607] [3.6870] [4.170] [4.1801] [TL] 4.285 4.401 4.201 4.0714.453 5.245 5.458 [*TL] [4.268] [3.637] [3.526] [4.034] [4.035] [4.782][4.899] [2ω] 117.1 115.6 114.9 117.7 101.7 98.6 99.4 [2δ] 52.7 79.2 78.652.3 60.5 48.3 46.1 [hF] 0.882 1.174 1.02 0.91 0.97 1.165 1.235 [*hF][0.878] [0.970] [0.856] [0.902] [0.879] [1.062] [1.109] [hR] 0.560 0.6760.779 0.723 0.683 0.642 0.642 [*hR] [0.558] [0.559] [0.654] [0.716][0.619] [0.585] [0.576] [IH] 1.004 1.2102 1.1914 1.009 1.1036 1.09691.1141Abbreviations are the same as those in the above examples. ([f]: focallength, [BL]: back length, [sum]: lens thickness, [TL]: total length,[2ω]: field angle (degree), [2δ]: exit angle (degree), [hF]: incidentheight, [hR]: exit height, [IH]: maximum image height, CE: comparativeexample)

As shown in the table 11, the maximum image height IH varies in a rangeof 1.004 to 1.2102. Although the maximum image height IH is smaller thanthe maximum image height 1.500 of the examples of the present invention,the total length TL of the optical system, the lens thickness “sum”, theincident height hF, and the exit height hR are greater than those of theexamples of the present invention. Hence, the total length TL and thelens thickness “sum” of the imaging optical system of each comparativeexample are large relative to the image size. Accordingly, the outerdiameter becomes significantly large. Thus, downsizing of the imagingoptical system is insufficient.

In the table 11, a value in parentheses [ ] shows a piece of the opticaldata standardized with the maximum image height IH. In each of thecomparative examples 1 to 7, the value “*TL” is within a range of 3.526to 4.899 and does not satisfy the expression (1). Thus, the total lengthof the optical system is not shortened sufficiently relative to the sizeof the image plane. The value “*hF” is greater than or equal to 0.85 anddoes not satisfy the expression (2). The value “*hR” is 0.558 at thesmallest and does not satisfy the expression (3), but the outer diameterof the fourth lens group G4 can still be reduced because the value onlyslightly exceeds the upper limit. In this case, however, the value “*hF”is 0.878, so that the outer diameter of the first lens group G1 cannotbe reduced sufficiently. Hence, the diameter of the entire imagingoptical system cannot be reduced. The value “*sum” exceeds 3.29, whichis large and does not satisfy the expression (5). Thus, the lensthickness is not reduced sufficiently.

The imaging optical system of the present invention has a large exitangle 2δ. The exit angle 25 in each table refers to an angle between apair of principal rays exiting from the surface of the fourth lens groupG4, proximate to the image plane, symmetrically about the optical axis15 a. Particularly, the exit angle 25 is the angle between the pair ofprincipal rays exiting toward the respective maximum image heightpositions, located symmetrically about the optical axis 15 a, on theimage plane IP. Hence, an angle between the principal ray directed toone of the maximum image height positions and the optical axis 15 a,that is, the angle of incidence of the principal ray on the maximumimage height position on the image plane IP is δ. The angle δ ofincidence is the maximum angle of incidence of the principal ray on theimage sensor 16.

As described above, photoelectric conversion efficiency of the imagesensor 16 decreases as the angle of incidence of the principal ray onthe image sensor 16 increases. Restrictions on the angle of incidence ofthe principal rays are significantly reduced by using the organic CMOSsensor as the image sensor 16. Hence, as described in each example ofthe present invention, the optical system is put into practical use evenif the maximum angle δ of incidence increases to a range of 35° to47.75°. When the maximum angle δ of incidence of the principal ray onthe image sensor 16 is rather small, the back-illumination type CMOSsensor can be used instead of the organic CMOS sensor. In most of thecomparative examples 1 to 7, the maximum angle δ of incidence of theprincipal ray is reduced to 23°-30°, which restricts the design of theimaging optical system.

The present invention has been described using the examples 1 to 10 eachwith the lenses only having spherical surfaces. Alternatively, theimaging optical system of the present invention includes one or moreaspheric surfaces. In addition to the use in the rigid tip section ofthe endoscope, the imaging optical system of the present invention canbe used in mobile information terminals such as mobile phones,stationary surveillance cameras, and vehicle-mounted cameras whichrequire reduction in size relative to the image size. The presentinvention can be implemented as the imaging devices having the imagingoptical systems with the above-described characteristics integrated withvarious types of image sensors.

Various changes and modifications are possible in the present inventionand may be understood to be within the present invention.

What is claimed is:
 1. An imaging optical system comprising: a negativefirst lens group; a positive second lens group; a positive third lensgroup; a negative fourth lens group, the first to fourth lens groups arearranged in this order from an object side; and an aperture stopprovided between the second and third lens groups; the imaging opticalsystem satisfying expressions (1) to (5):2.00<TL/IH<3.00  (1)0.37<hF/IH<0.5  (2)0.37<hR/IH<0.5  (3)3.5<|f1/IH|<4.5  (4)1.8<sum/IH<2.1  (5) wherein “IH” represents a maximum image height, “TL”represents a total length of “sum” and a back focal length, the “sum”represents lens thickness of the entire imaging optical system, “hF”represents an incident height of a principal ray, corresponding to themaximum image height “IH”, at a surface proximate to the object side,“hR” represents an exit height of the principal ray at a surfaceproximate to an image plane, and “f1” represents a focal length of thefirst lens group.
 2. The imaging optical system of claim 1, wherein thefirst lens group is a single lens with a concave surface facing theimage plane, and the fourth lens group is a single lens with a concavesurface facing the object side, and at least one of the second lensgroup and the third lens group is a doublet made up of a positive lenselement and a negative lens element attached together.
 3. An imagingdevice comprising: an imaging optical system of claim 1; and an imagesensor disposed on an image plane of the imaging optical system, theimage sensor being a back-illumination type CMOS image sensor or anorganic CMOS image sensor.
 4. An imaging device comprising: an imagingoptical system of claim 2; and an image sensor disposed on an imageplane of the imaging optical system, the image sensor being aback-illumination type CMOS image sensor or an organic CMOS imagesensor.
 5. The imaging device of claim 3, wherein the imaging device isincorporated in a distal end portion of an endoscope for imaging a bodycavity.
 6. The imaging device of claim 4, wherein the imaging device isincorporated in a distal end portion of an endos cope for imaging a bodycavity.
 7. The imaging optical system of claim 1, wherein2.211<TL/IH<3.00 is satisfied.